This invention relates to the separation and purification of C-4 hydrocarbons, particularly butadiene-1,3, from C-4 hydrocarbon mixtures utilizing extractive distillation with a selective solvent. It pertains to such extractive distillation when using a solvent composition containing as alkoxynitrile as an essential component and is specifically directed to modifying the solvent composition by including therein a combination of substances which function in a synergistic manner to prevent polymer formation during the extractive distillation operation.
It is well known to the art that C-4 hydrocarbon mixtures containing C-4 hydrocarbons of different degrees of unsaturation, such as mixtures of butadiene-1,3 with butanes (including n-butane and isobutane) and butylenes (including butene-1, and cis- and trans-butene-2) which are not easily separable by ordinary fractional distillation because of similarities in boiling points and azeotrope formation are much more efficiently separated into their individual components by the process of extractive distillation with a solvent of relatively higher boiling point which selectively dissolves the more-unsaturated butadiene-1,3 component. In the extractive distillation process as conventionally carried out, the selective solvent is introduced near the top of a distillation column and flows down the column as the distillation proceeds where it is contacted with the vapors of the hydrocarbons as they travel up the column. The more saturated hydrocarbons not dissolved by the solvent go overhead while the bottoms of the column contain the solvent plus the more unsaturated butadiene-1,3 component, which is removed from the solvent in a stripping column, or by other suitable means, and the lean solvent is recirculated to the column.
While a large number of selective solvents are known which can be used in the extractive distillation process, certain solvent compositions containing an alkoxynitrile are desirably used for the separation of butadiene-1,3 from C-4 hydrocarbon mixtures, because of their high degree of selectivity for butadiene-1,3 and their compatibility with existing plant configurations, provided the solvent composition preferably contains a substance, known as an inhibitor, which largely prevents the butadiene-1,3 in the solvent, necessarily exposed to elevated temperatures of the order of 80.degree. to 160.degree. C. or higher, from polymerizing to form polymer the presence of which results in fouling and plugging of equipment so that continuous operation over an extended period of time is difficult or impossible. The inhibitor should not itself polymerize or form an adduct with butadiene-1,3, as does furfural, and hence the known solvent compositions containing alkoxynitrile and furfural are not suitable.
My copending application Ser. No. filed concurrently herewith on, the disclosures of which are incorporated herein by reference, teaches means of modifying the known selective solvent compositions containing an alkoxynitrile to render them more suitable and, at the same time, to enable them to function in the extractive-distillation-purification of butadiene-1,3 without substantial formation of butadiene-1,3 polymer. It teaches, inter alia, that 2,4-dinitrophenol (DNP) is a unique inhibitor for butadiene-1,3 polymerization when present with an alkoxynitrile in a selective solvent composition.